I disagree that yeast slurry shouldn't be 'washed' (purified, really) in general. I think it's all about how you go about it and what you want to accomplish. If it's done right, in a sanitary manner, it's perfectly safe and will allow more consistency and predictability in your finished beer. Storing under cold water over long periods is better than beer for yeast health, too. Personally, I don't like the idea of generations-old trub and hops going from brew to brew, but I'm overly paranoid about stuff like that most of the time anyway.
But the real issue here is the work-to-reward ratio. Basically, do you want to go through all this trouble when you won't see any real, appreciable improvement in quality? Because you probably won't. It's one of those things where, at home, on the five or 10 gallon level, there are so many other confounding factors to consistent beer that this alone won't make the difference. If you have, say, physiologically healthy yeast, predictable pitch rates, wort compostion and oxygenation, reliable temperature control, etc., not storing and/or pitching crap in with your yeast becomes important to consistent outcome. But unless you have those other things under control, you just won't see a difference that wouldn't be within the limits of variability anyway.
So I hate to see what I'd consider slurry purification discounted in general, though I do agree it's entirely dependent on what exactly you want to do with your brews.